Phototube



April 20, 1937. a. SALZBERG PHOTOTUBE Filed NOV. 14, 1936 mvENfo BERNARD SALZBERG WM TTQRNEY Patented Apr. 20, 1937 UNITED STATES PHOTOTUBE Bernard Salzberg, East mesne assignments, America, New York, Delaware Orange, N. J., assignor, by to Radio Corporation of N. Y., a corporation of Application November 14, 1936, Serial No. 110,805

4 Claims.

My invention relates to light sensitive electron discharge devices, more particularly to phototubes in which secondary emission amplification is produced.

In the conventional phototube making use of secondary emission amplification, a light sensitive electrode or photocathode which releases photoelectrons when exposed to light is oppositely disposed to a secondary emitting electrode or auxiliary cathode coated with material which releases secondary electrons when struck by the photoelectrons from the photocathode. A rod or grid-like collecting electrode or anode is placed between the photocathode and the auxiliary 5 cathode. The usual arrangement of the electrodes and the usual shape of the electrodes do not promote optimum characteristics from the standpoint of obtaining maximum light on the photocathode, of preventing useless dissipation of both photoelectrons and secondary electrons,

of obtaining high secondary emission and collecting the maximum number of secondary electrons at low collector voltages and obtaining low interelectrode capacities.

To obtain. an efiicient secondary emission amplification phototube with the most desirable characteristics the electrodes should be so constructed and positioned that as much of the available light as possible reaches the photocathode and so that as much of the photoelectron emission as possible strikes the secondary emitter cathode, that is so that as little of it is diverted or intercepted by the collector as possible. To obtain good characteristics it is also necessary that the greatest possible ratio of secondary electrons to primary electrons at the secondary emission electrode or cathode is obtained and that as much of the secondary emission current as possible is collected with a minimum voltage at the collector or anode. It is also desirable to reduce the capacitance between the electrodes.

The object of my invention is to provide an improved type of secondary emission amplification phototube which satisfies the above conditions for an efiicient phototube.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a perspective with the envelope partially broken away of a phototube made in accordance with my invention, and Figure 2 is a transverse section taken along the line of 2-2 in Figure 1.

One embodiment of a phototube made in accordance with my invention comprises an envelope l6 provided with the usual base II and stem 12. Supported from the stem [2 is a conventional semicylindrioal or arcuate shaped photocathode l3 which may be coated on its inner or concave surface with any conventional light sensitive coating, such as caesium for example.

In accordance with my invention I dispose oppositely to the photocathode I3 and preferably coaxial therewith a secondary emitting electrode l4 also preferably semi-cylindrical or arcuate shaped, although it could be fiat, and coated on its convex surface which is first oxidized and then coated with an alkali metal film, for example caesium, this coated convex surface being oppositely disposed to the concave surface of photocathode [3. A shiny surface also provides a good secondary emitting surface. Mounted parallel to either edge of the secondary emitting electrode 14 but outside of any line normal to its surface are a pair of coextensive rectangularly shaped collector electrodes or anodes I5 and IS. The secondary emitter electrode I4 is maintained at a positive potential with respect to the photocathode I3 by means of the voltage I1 and the collector electrodes l5 and i6 are in turn maintained positive with respect to the secondary emitting electrode M by means of the source of voltage l8.

As a result of the electrode shape and field distribution between electrodes the photoelectrons are focused into a beam from the photocathode onto the emitting surface of the secondary emitting electrode I4 and are not dispersed. Because the collectors l5 and I6 are disposed outside the focus of the beam from the photocathode they do not interfere materially with the stream of photoelectrons to the secondary emitting electrode 14. The anodes are sufliciently close to the secondary emitter cathode so that it is not necessary to use a very high voltageto overcome the space charge drop caused by the secondary electrons and to collect the secondary electrons from the secondary electrode. Because the secondary emitter cathode surface is convex space charge is diffused and the secondary emission current saturates sharply, that is a lower collector voltage is needed to obtain saturation or maximum collector current than in case the surface of the secondary emitter electrode were reversed so that its concave surface faced the concave surface of the photocathode and was coated on its concave surface with the emitting material. The distance between the photocathode and the collector is made as small means limited to the exact form illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. A phototube including an envelope containing a photocathode, an oppositely disposed secondary emitting electrode and a collector electrode adjacent said secondary emitting electrode but outside any line normal to the emitting surface of said secondary emitting electrode.

2. A phototube including an envelope containing an arcuate shaped photocathode having a light sensitive coating on its concave surface and an oppositely disposed secondary emitting cathode having its surface opposite the'concave surface of the photocathode coated with emitting material, and a collector electrode adjacent the edge of the secondary emitting electrode but outside a line normal to the emitting surface of said secondary emitting electrode.

3. A phototube including an envelope containing an arcuate shaped photocathode having its concave surface coated with light sensitive material, an oppositely disposed arcuate shaped sec ondary emitting electrode having a convex surface coated with electron emitting material and collector electrodes adjacent the edges of the convex surface of said secondary emitting electrode outside a line normal to said convex surface.

4. A phototube including an envelope containing a semi-cylindrical photocathode coated with light sensitive material on its concave surface, a semi-cylindrical secondary emitting electrode coaxial with said photocathode and having its convex surface oppositely disposed to the concave surface of said photocathode and coated withan electron emitting material and a pair of rectangular plates parallel to the longitudinal edges of the semi-cylindrical secondary emitting electrode and coextensive therewith outside of any line normal to the convex surface of said secondary emitting electrode.

BERNARD SALZBERG. 

